出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2012/07/15 00:30:46」(JST)
A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes.
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Biological membranes consist of a phospholipid bilayer and a variety of proteins that accomplish vital biological functions.
Membrane proteins can be divided into several categories:[1]
In addition, pore-forming toxins and many antibacterial peptides are water-soluble molecules, but undergo a conformational transition upon association with lipid bilayer and become reversibly or irreversibly membrane-associated.
A slightly different classification is to divide all membrane proteins to integral and amphitropic.[2] The amphitropic are proteins that can exist in two alternative states: a water-soluble and a lipid bilayer-bound. The amphitropic protein category includes water-soluble channel-forming polypeptide toxins, which associate irreversibly with membranes, but excludes peripheral proteins that interact with other membrane proteins rather than with lipid bilayer.
Integral membrane proteins are permanently attached to the membrane. They can be defined as those proteins which require a detergent (such as SDS or Triton X-100) or some other apolar solvent to be displaced. They can be classified according to their relationship with the bilayer:
Peripheral membrane proteins are also known as extrinsic proteins, they do not interact with the hydrophobic core of the lipid bilayer. Some peripheral membrane proteins are located at the outer part of the plasma membrane (exoplasmic). They interact with the membrane indirectly by binding to the integral membrane proteins.
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Peripheral membrane proteins are temporarily attached either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations.
Integral and peripheral proteins may be post-translationally modified, with added fatty acid or prenyl chains, or GPI (glycosylphosphatidylinositol), which may be anchored in the lipid bilayer.
Polypeptide toxins, such as colicins or hemolysins, and certain proteins involved in apoptosis, are sometimes considered a separate category. These proteins are water-soluble but can aggregate and associate irreversibly with the lipid bilayer and form alpha-helical or beta-barrel transmembrane channels.
Proteins are specifically targeted to many different types of biological membranes [4]
Membrane proteins commonly function as complexes. These complexes are vital to cellular function. Understanding how these complexes are assembled, degraded, and their composition are crucial to understanding their function and regulation. Reoccurring in recent literature are the ideas that: membrane protein complexes assemble in an orderly fashion, chaperones aid assembly by preventing unfavorable interactions, and membrane proteins can be interchanged in existing complexes. Membrane protein complexes assemble through the orderly assembly of intermediates. For example, the simple membrane-embedded four subunit complex, cytochrome bo3 of Escherichia coli, is assembled via two intermediate complexes. This suggests a linearly organized assembly pathway. Although interactions between other subunits could lead to the formation of many intermediates, they do not occur. Ordered assembly could be the cell's protection against harmful intermediates. Chaperones interact with membrane proteins guiding their assembly. They aid in preventing the assembly of dead-end and toxic intermediates, as well as unwanted aggregations. Via chaperones assembly can occur through inactive intermediates potentially preventing damaging interactions they could cause. Membrane protein complexes are not fixed entities. Though a process called dynamic exchange, membrane proteins are exchanged in and out of exsitisting protein complexes. This has its implications as a repair mechanism and in regulation. [5]
There are two different types of membrane proteins depending upon the synthesis it undergoes and they are Constitutive membrane protein and non-constitutive membrane protein.
The messengerRNA attaches to the translocon which is located in the cell membrane. The mRNA is translated into the translocon transmembrane tunnel. After the synthesis of protein the mRNA is released which closes the translocon and the protein is released in the bilayer membrane. When the protein is left in the membrane bilayer more protein folding occurs creating its final 3D structure (White).
Examples of non-constitutive membrane proteins are toxins and antimicrobialpeptides. These membrane proteins are inserted into specific target membranes by physicochemical processes. It is usually inserted before, during, or after oligomerization into the target membrane (White).[6]
In membrane proteins the two known structural classes of membrane proteins are alpha-helical bundle and beta-barrel porin. The portion of the membrane proteins that are attached to the lipid bilayer are consisting of hydrophobic amino acids only. This is done so that the peptide bonds' carbonyl and amine will react with each other instead of the hydrophobic surrounding. The portion of the protein that is not touching the lipid bilayer and is protruding out of the cell membrane are usually hydrophilic amino acids.[6]
The structures of membrane proteins are stabilized by weak interactions and influenced by additional interactions with the solubilizing environment. The influence of the environment on membrane protein structures is especially significant. Despite the significant functional importance of membrane proteins, the structural biology has been particularly challenging as shown by the low number of membrane protein structures determined. Integral membrane proteins are present in a heterogeneous environment that poses major obstacles for existing structural methodologies.
Many of the successful membrane protein structures are characterized by X-ray crystallography and are very large structures in which the interactions with the membrane mimetic environments can be anticipated to be small in comparison to those within the protein structures. The small domains are particularly sensitive to the influence of membrane mimetic environments, potentially leading to non-native structures. Fortunately, there are many sample preparation conditions that can be chosen for crystallization and for solution NMR. All membrane protein structural biology should be subjected to careful scrutiny; through a combination of structural methodologies it should be possible to achieve an understanding of the native functional state for membrane protein structures.[7]
White, Stephen. “General Principle of Membrane Protein Folding and Stability.” Stephen White Laboratory Homepage. 10 Nov. 2009. web.
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